Last I heard they were booking until August 14th.
When I first wanted to book they were only booking through mid May. I would call every Monday and Thursday to ask if they extended it yet. They always asked when we were coming, I wonder if they keep track and use that as a decision to extend the dates.
About 3 weeks after I started calling and asking about the beginning of August they opened up reservations until Mid August. So keep calling and checking.